1. Field of the Invention
The present invention relates to a piezoelectric resonance device and, more specifically, to an energy trapping type piezoelectric resonance device utilizing the mode of thickness extensional vibration.
2. Description of the Background Art
U.S. Pat. No. 3,384,768 discloses an energy trapping type piezoelectric-resonator utilizing the mode of thickness extensional vibration, which is prepared from Pb (Zr, Ti)O.sub.3 (PZT) piezoelectric ceramics. This energy trapping type piezoelectric-resonator is formed of a piezoelectric ceramic plate and electrodes are formed on both sides of the piezoelectric ceramic plate, with each of the electrodes having a smaller area than the piezoelectric ceramic plate. The piezolelectric ceramic plate is formed of a material such as PZT system having an effective Poisson's ratio of at least 166 . Utilizing a material having an effective Poisson's ratio of less than 1/3, it is impossible to obtain a frequency lowering type energy trapping piezoelectric resonance device using the fundamental thickness extensional mode.
It is desirable to improve the aforementioned piezoelectric-resonator, for use in a higher frequency range. In order to vibrate such a piezoelectric-resonator in a higher frequency range, the piezoelectric ceramic plate may be reduced in thickness. However, the lower limit of the thickness is about 200 .mu.m in view of handling problems, and it is difficult to further reduce the thickness in practice. Thus, the actual frequency range that has been attained with the thickness fundamental vibration mode has been not more than 10 MHz.
On the other hand, it may be possible to use a piezolelectric-resonator in a higher frequency range by utilizing the third harmonic of thickness mode vibration. However, the response level of the third harmonic is considerably lower than that of the fundamental vibration, so the third harmonic has not generally been usable in the past.
Further, in a conventional piezoelectric-resonator utilizing the mode of thickness extensional vibration, it has been impossible to trap vibration energy by using a frequency lowering method unless the material thereof has an effective Poisson's ratio of at least 1/3. Therefore, only a limited number of materials have been usable.
In addition, in a conventional piezoelectric-resonator, the energy is not well trapped and it has been impossible to fully eliminate spurious outputs superimposed on the resonance waveform. Therefore, a low level of spurious outputs, i.e. ripples, in this frequency range have prevented the utilization of such a device as a filter, and as to its utilization as an oscillator, it had the disadvantage that its Qm becomes lower.